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Integrated circuits and devices that can handle and control significant currents are increasingly being developed. The packaging of these integrated circuits and devices usually needs to be hermetically sealed. The leads used in such packages are highly conductive and the body of the package is made of a good insulator. Highly conductive materials have a thermal coefficient of expansion (TCE) that is much higher than that of insulators. As the heat being generated by such packages increases, the bonds between the leads and the insulator are stressed. Such stresses can lead to a shortened life of the package due to loss of the hermetic seal.
Technologies have been developed to bridge the gap in TCEs and maintain the hermetic seal needed for reliable operation. Among the technologies tried have been the development of less conductive leads having a TCE more nearly matching that of the insulator, insulative materials with a higher TCE for use as the insulator and development of multi-step sealing mechanisms that progressively span the TCE gap between the two materials being bonded. Unfortunately, these methods have resulted in compromised performance, expensive materials or time consuming and expensive processes in the manufacturing cycle.
The invention provides a lead structure having a lead of low resistance material disposed within a surrounding sleeve or collar of low expansion material which is bonded at one end to the lead. The sleeve or collar is bonded on its outer surface to an insulating wall through which the lead structure extends. The lead is preferably copper and the sleeve or collar is preferably a nickel-alloy such as Kovar. The lead is hermetically sealed to the surrounding sleeve, and the sleeve is hermetically sealed to the insulating wall to provide a hermetic structure which does not detract from the use of high conductivity electrical lead materials which are often not employable in conventional hermetic sealed leads or packages. A plurality of lead structures can be employed in one or more walls of a circuit package.
The low expansion sleeve has a thermal coefficient of expansion (TCE) sufficiently similar to that of the insulator wall to which it is bonded to provide a hermetic seal that is resistant to thermal stresses. The sleeve is bonded to the insulator by brazing or other known hermetic sealing techniques. The lead is bonded to the surrounding sleeve by welding or other known techniques sufficient to withstand the thermal stresses experienced during expansion and contraction of the lead relative to the sleeve.